In general, shrink films can be categorized as conventional shrink films or specialized shrink films. “Conventional shrink films” are packaging films used to collate groups of articles into a unitized package for the purpose of handling, shipping, distribution and final sale. Conventional shrink films are typically manufactured using blown film processes; both monolayer and multilayer films are commonly produced. In general, conventional shrink films shrink anisotropically, e.g., films may shrink 65 to 85% in the machine direction (MD) and −10 to 20% in the transverse direction (TD). Higher shrinkage in the MD is used advantageously by circumferentially wrapping articles such that the shrink film's MD direction is oriented in the circumferential wrapping direction. Non-limiting examples of articles that are frequently wrapped with conventional shrink film include: canned soup, canned fish, bottles and cans of beverages such as water, soft drinks, beer, energy drinks, liquid protein drinks and multipacks of bottles or cans. In some cases multiple articles (e.g., 12 or 24 bottles) contained within a tray are wrapped with shrink film and after passage through a shrink tunnel a secure unitized package is produced that can be easily handled and shipped. Any suitable material may be used to construct the tray, e.g., cardboard or plastics. Rather than a tray, a case may be used to contain the multiple articles and the case shrink wrapped. In addition loose articles may be shrink wrapped, i.e., a tray or case is not employed. In some cases, the unitized package (post-shrink state) is not completely wrapped with shrink film, i.e., the package has an open end which is not encapsulated with shrink film, e.g., the side-opening in a case of shrink wrapped bottle water. In other cases, the unitized package is completely encapsulated with shrink film. In some applications the shrink film may be pigmented and/or printed such that a high-quality image is displayed for advertising and/or promotional purposes. Conventional shrink films benefit from the optimal selection of ethylene polymer and/or an optimal blend of the most appropriate ethylene polymers; non-limiting examples of ethylene polymers include plastomers, very-low density polyethylene (VLDPE), linear-low density polyethylene (LLDPE), medium density polyethylene (MDPE), high density polyethylene (HDPE), high pressure low density polyethylene (LDPE) and copolymers of ethylene and vinyl acetate (EVA). In this disclosure, plastomers, VLDPE, LLDPE, MDPE and HDPE are “ethylene polymers”, i.e., macromolecules comprising ethylene and optionally one or more comonomers and manufactured using any catalyst or polymerization process. Conventional shrink films may have a monolayer or multilayer construction; non-limiting examples of multilayer films include films containing 2 to 11 layers where the chemical composition of each layer may be the same or different.
In addition to conventional shrink film, a second category of shrink film is specialized shrink film. “Specialized shrink films” are manufactured such that the film's shrink properties fall within specific limits; non-limiting examples of properties include MD % shrink, TD % shrink, MD shrink force, TD shrink force, film clarity or film sealability, etc. Specialized shrink films are manufactured by non-conventional techniques, non-limiting examples include double-bubble processes, the triple-bubble processes or tenter frame processes. Frequently, in the film and packaging industries, specialized shrink films are also called “biaxially oriented shrink films”. A non-limiting example of a specialized shrink film is a film that shrinks about 50% in the MD and about 50% in the TD; after shrinking, such a film produces a smooth and uniform wrapping on articles of various shapes; non-limiting examples of shapes include the wrapping of square, rectangular or round consumer goods, e.g. CD cases, books, magazines, boxes, etc. Specialized shrink films may also be used to shrink wrap large items; non-limiting examples include pallets, industrial equipment, cars, or boats, etc. Specialized shrink films are also used to produce smooth and uniformly wrapped food products; non-limiting examples include cheese, red meat and poultry, etc. Specialized shrink films may have a monolayer or multilayer construction; non-limiting examples of multilayer films include films containing 2 to 11 layers. Generally, multilayer films contain chemically distinct layers that impart additional characteristics; non-limiting examples of characteristics include water vapor resistance, oxygen barrier resistance or puncture resistance. Specialized shrink films benefit from the optimal selection of an ethylene polymer and/or an optimal blend of the most appropriate ethylene polymers; non-limiting examples include, plastomers, very-low density polyethylene (VLDPE), linear-low density polyethylene (LLDPE), medium density polyethylene (MDPE), high density polyethylene (HDPE), high pressure low density polyethylene (LDPE) and copolymers of ethylene and vinyl acetate (EVA). Typically, in specialized shrink films, high pressure LDPE is used less frequently or in lower concentrations relative to conventional shrink films. A variety of processes can be used to apply a specialized shrink film to the article to be wrapped and to heat/shrink wrap the film around the article(s); non-limiting examples of the latter include a heated shrink tunnel, hot water bath, steam bath, etc.
There is a constant need to develop improved shrink films. Non-limiting examples of desirable improvements in shrink films include: specific film shrinkages (%) in the machine and transverse direction, a specific film holding force, improved film tensile strength (after shrinkage), improved load retention, acceptable film stiffness, improved film puncture resistance and/or film toughness, specific film optical properties and acceptable film sealing properties. Film optical properties such as low haze, high gloss and clarity may (or may not) be important depending on the application, e.g., low haze, high gloss and high clarity are desired if the product must be clearly visible though the shrink wrap film or if the shrink film is printed and the printed image functions as a product advertisement.
Disclosed herein, shrink films comprising at least one heterogeneous ethylene interpolymer, made using an in-line Ziegler-Natta catalyst, having a Type II pseudo-flat comonomer distribution profile has improved properties; for example, improved puncture and machine direction shrink tunnel shrink force; relative to comparative shrink films comprising at least one heterogeneous ethylene interpolymer, made with a conventional Ziegler-Natta catalyst, having a conventional Type IV comonomer distribution profile.